Speculators would recognize it and attempt to profit by front-running the government operations (increasing the cost to the government, and dampening the volatility).
Bitcoin service providers could offer volatility-protection (instant conversion to other currencies), as some do already.
Governments could lose money on each manipulated boom-bust cycle, and at the bottom of each cycle, Bitcoin would still be alive and ready for new uses.
Far more likely, in my mind, is an attempt to coopt Bitcoin. Officially approve it, with reporting/identity conditions that don't encumber legal use but ensure tax collection. Or, launch a Bitcoin-like competitor backed by government redemption guarantees (T-Bills, TIPS, etc). So, above-ground businesses can get most of the crypto-currency benefits without the rough edges created by its most anti-State qualities.